In brake boosters of the prior art the valving required to control fluid communication through the booster is generally precision machined with close tolerances so that contamination of the booster fluid severly hinders the operation of the booster. For example, spool valves which are slidably mounted within stepped bores are easily wedged therein if foreign matter or particles are carried in the booster fluid. Consequently, it is believed to be an advance in the state of the art to provide a booster which is economical to manufacture and substantially independent of fluid contamination.